1. Field of the Invention
The present invention relates to an active matrix type picture image display device and a method of driving the same in which a light emitting operation of an electro optical element of which emission is caused by flowing a drive current through a light emitting thin film such as an organic semiconductor film is controlled by switching elements such as thin film transistors (hereinbelow will be referred to as TFTs).
2. Conventional Art
In response to coming of a highly information oriented society, demands of such as personal computers, car navigation devices, portable information terminals, information communication products and combined devices thereof are increasing these days. For these products thin and light weighted displays with low electric power consumption are suitable, therefore, liquid crystal display devices or display devices including self light emitting type electro optical elements such as electro luminescence elements (hereinbelow will be referred to as ELs) and light emitting diode elements (hereinbelow will be referred to as LEDs) are how used.
Since the latter display devices including the self light emitting type electro optical elements show such advantages as of high visibility, broad visual angle and high response characteristics which are suitable for displaying motion pictures, the latter display devices are recognized particularly as suitable in the future, because video image display will become major importance. In particular, a rapid improvement in light emitting efficiency in these days of organic EL elements and organic LED elements (hereinbelow will be called inclusively as OLEDs) using an organic substance as a light emitting layer and development in network technology which permits video image signal communication in combination increase more and more an expectation for OLED displays.
In order to enhance power efficiency in OLED displays an active matrix drive by means of thin film transistors is effective which will be explained later. Art in which an OLED display is constituted in an active matrix structure and is driven therewith is, for example, disclosed in JP-A-4-328791 (1992), JP-A8-241048 (1996) and U.S. Pat. No. 5,550,066, and further, art relating to a drive voltage of such OLED display is, for example, disclosed in PCT application laid-open No. W098/36407.
In a typical pixel in an OLED display the light emitting intensity of an OLED element is controlled by an active matrix element drive circuit which is constituted by two TFTs (one is for a switching transistor and the other is for a driver transistor) and one capacitor. Pixels are arranged near respective cross points of m lines and n rows matrix which is formed by n pieces of data lines to which picture image signals are fed and m pieces of scanning lines (or gate lines) to which scanning signals are fed.
In order to drives the pixels, through successive application of scanning signals (or gate voltages) onto the gate lines of m lines, the respective switching transistors are turned on to complete scanning in vertical direction once in one frame period Tf, and a turn on voltage is again applied to the first gate line.
In the above pixel drive scheme, an interval when the turn-on voltage is applied on a single gate line gives below Tf/m. As a value for one frame period Tf about {fraction (1/60)} sec is generally employed. At the time when a turn-on voltage is being applied on a certain gate line, all of the switching transistors connected to the certain gate line are rendered into a on state, and in synchronism therewith picture image signals (data voltages) are applied at the same time to data lines of n rows, which is called as line-at-a-time scanning method and commonly used in the field of active matrix type liquid crystal display devices.
The data voltage is stored in the capacitor during the time when the turn-on voltage is applied to the gate line and is kept there at substantially the same value during one frame period. The voltage value at the capacitor defines the gate voltage of the driver transistor, thus the value of current flowing through the driver transistor is regulated so as to flow a predetermined current through the OLED element and to cause light emission. Response time of an OLED element of starting light emission after a voltage is applied is usually below 1 xcexcs, therefore, the OLED element can be operated so as to follow a swift motion picture image (motion picture).
Now, with the active matrix drive a high efficiency is realized, because the light emission is effected during the entire one frame period. When comparing this active matrix drive with a simple matrix drive in which an OLED element is driven by connecting the respective diode electrodes thereof to a vertical scanning line and a horizontal scanning line without providing the TFTs, the difference of the efficiency is significant.
In the simple matrix drive, the current flows through the OLED element only the period when the vertical scanning line is selected, therefore, in order to obtain a brightness equivalent to a light emission in the one frame period only with such a short period light emission a light emission intensity multiplied by about vertical scanning line number is necessitated in comparison with that for the active matrix drive. For fulfilling such requirement the drive voltage and the drive current therefor have to be inevitably increased, which increases loss power consumption such as heating and can not prevent power efficiency reduction.
As will be apparent from the above, it is considered that the active matrix drive is superior over the simple matrix drive with regard to the lower power consumption. Because of the high speed response characteristics of the OLED elements it has been believed that such OLED elements are suitable for motion picture displays. However, the active matrix drive for such OLED elements according to the conventional art employs the same drive method as for the liquid crystal displays (LCDs), namely a hold type display method, in which the respective pixels are displayed, in other words the respective OLED elements are caused to light-emit, over one frame period.
Hidekazu Ishiguro et al. xe2x80x9cConsideration on Motion Picture Quality of the Hold Type Display with an Octuple-rate CRTxe2x80x9d (Technical Report of The Institute of Electronics, Information and Communication Engineers, EID 96-4(1996-06), pp19-26) discloses that it is unavoidable in a LCD of such phenomenon that edges of a moving body are blurred during motion picture display due to the hold type display method.
Although the problem of blurred edge of motion picture images was pointed out with regard to the LCDs, the cause of the blurred edges is due to the hold type display method. Therefore, the same problem of blurred edges of motion picture images likely arises with regard to the active matrix driven OLED elements when such OLED elements are operated with the hold type display method.
As has been mentioned above, in the conventional art the blurred edges phenomenon during motion picture display when electro optical elements such as OLED elements are driven by the active matrix circuits was not taken into account which causes deterioration of the picture qualities thereof.
An object of the present invention is to provide a picture image display device and a method of driving the same which suppresses a blurred edge during motion picture display, when the electro optical elements included therein are driven through active matrix circuits, and improves the picture quality thereof.
A feature of the present invention is to drive pixels by interposing a quenching period in which the electro optical elements are quenched after scanning a plurality of gate lines for displaying one picture image. In other words, the present invention drives the pixels while forming the quenching period in which the electro optical elements are once quenched between one frame and the subsequent frame thereto.
In a preferred embodiment according to the present invention, the pixels are driven while forming a quenching period in which the electro optical elements therein are quenched within one frame period for displaying one picture image.
In the present invention, since the pixels are driven in such a manner that a quenching period of quenching the electro optical elements is formed after scanning a plurality of gate lines for displaying one picture image, integration of white background or background signals is eliminated at the time of line of sight movement during the quenching period, thus the blurred edges is prevented, in other words the display characteristics of motion pictures are greatly improved.